Helen Mirren sits very straight, folding and unfolding her hands from a resting position just below her bosom as she speaks. It is a gesture reminiscent of Her Majesty the Queen.

In all other respects, the 67-year-old actress, who possesses a glamour that time has not withered, bears little resemblance to the 86-year-old monarch either now or at any point in her 60-year reign.

Yet ever since she played the title role in The Queen – the 2006 film directed by Stephen Frears and written by Peter Morgan – the two have become muddled in people’s minds, making the Queen seem slightly more beautiful than she is and Mirren rather more regal.

That confusion will become even greater next Friday when Mirren steps on to the stage as Elizabeth II in The Audience, Morgan’s new play which examines the monarch’s relationship with the prime ministers who have served her. Morgan wrote it with Mirren in mind – but she was initially reluctant to reprise the role.

“I didn’t want to do it. The first thing I emailed to him was ‘You b------’,” she says. “[As an actress] you want to go forward and not be stuck with a character for the rest of your life. I always think of the obituary, you know?” She is laughing now. “Well, I do. It was Prime Suspect for ages: ‘Jane Tennison has been sadly knocked over by a bus’.” Now the image of Mirren as the hard-drinking detective she played for seven series on ITV has been supplanted in people’s minds by her uncanny portrayal of the Queen.

Helen Mirren dressed as the Queen in the new play, The Audience. Photo: Johan Persson

We are talking during a break in rehearsals for the new play. Harold Wilson (played by Richard McCabe) has just wandered out of the room; David Cameron (Rufus Wright) has not yet arrived. Around a table, surrounded by books, papers and scripts, sit Mirren, Morgan and the director Stephen Daldry. All are smiling and apparently relaxed, despite the fact that they are only two weeks from the opening of one of the most anticipated plays of the season.

Morgan first came up with the idea for The Audience when he was working on The Queen, which was set immediately after the death of Diana, Princess of Wales. “I remember people were keen that I didn’t have Blair in the film because they felt that would make it less filmic,” he says. “But I really loved writing the Blair-Queen scenes because that was where it came alive for me.”

He was particularly fascinated by “the audience” – the confidential weekly meetings that have taken place, since Churchill’s day, between the monarch and her first minister – and decided that one day he would return to the theme. “I was prepared for Helen to say no,” he says, adding, deadpan, “You know, there are better actresses out there, and people I have been longing to work with...”

Mirren laughs, but becomes serious when she explains that she had resolved to turn the part down. “I was absolutely tortured about it,” she says. “I went to the first read-through thinking ‘I am not going to do this. I can’t go back to that.’ And then I walked into that room and there was Peter and there was Stephen and there was Robert Fox [the producer] and Bob Crowley [the designer]. I looked at that line-up and I thought you’re an idiot if you don’t do this. You need your head examined. These are among the very best people in British theatre and they are offering you a role. You should be so lucky.” She changed her mind on the spot. And after all, she says, the piece is so different from The Queen “that it feels like a new game, really”.

All three are coy about exactly what will unfold on stage. Twelve prime ministers have served the Queen, but only seven are currently on the cast list: we know that Haydn Gwynne is playing Mrs Thatcher, and Nathaniel Parker is Gordon Brown, but no one has so far been announced as Tony Blair. Does that mean he isn’t appearing?

“One of the great joys about a new play is you don’t know what is going to come at you,” says Daldry, with a grin. “To be honest, Peter hasn’t made his mind up yet. So we’re not quite sure.”

It is hard to imagine that things are quite as fluid as that, but for Mirren one of the pleasures of this process is working on a new play. Somewhat surprisingly, given the variety of a career which has ranged from playing Shakespeare’s Cleopatra on stage to a gangster’s girl in The Long Good Friday, it is only the second stage premiere she has ever appeared in. The first was her memorable performance as a self-destructive rock star in David Hare’s Teeth ’n’ Smiles, in 1975, at the very start of her career.

She says she relishes “the amazing, organic process of having the writer in the room, which Peter has been all day, every day”. He has been open to input from the cast, as each actor turns up with their own ideas about the real historical figure they are playing – from Robert Hardy, who is taking on the role of Churchill for the eighth time, to Paul Ritter, who tackles the much less theatrically familiar figure of John Major.

All the actors, Morgan jokes, have become obsessed with their characters: “They go away and do major postgraduate work on it and come back as biographers and experts…” And, adds, Miren, “in love” with them. “You have to be, because you have to understand them and in that process you have to love them.”

The most obvious danger, then, is that instead of The Audience being a play, it becomes a series of impersonations. Daldry dismisses this idea quite sharply. “It’s not a Mike Yarwood show. The actors are playing the scenes,” he says. Morgan, on the other hand, admits that the thought has haunted him. “I lie awake in a cold sweat about it because there is no single story. It is like being on the top of a high building, throwing yourself off, and having a good look through each window as you are falling. By the time you hit the ground you have a good composite idea of life inside that tower block.”

His aim is, first and foremost, to give audiences a good night out. But there is a serious purpose beneath these imagined conversations between a monarch and her subjects. “As soon as you see the Queen and the prime minister, it somehow goes to the core of what it is to be one of us,” he says. “It’s a play about being British and how our country is put together.” Daldry adds, “And the nature of the constitutional monarchy. What exactly is the relationship between state and crown and how has that been negotiated down the years. It’s fascinating.”

Since The Queen, for which Mirren won the an Oscar, perceptions of the monarchy have shifted yet again – most crucially thanks to the Queen’s appearance, playing herself opposite Daniel Craig’s Bond, in Danny Boyle’s opening ceremony film.

Mirren was as delighted as everyone else to see the actual queen usurping ‘her’ role. “I absolutely loved it. Stephen had said beforehand – because we were beginning to talk about this – I’m going to ask the Queen and I thought, I know the Queen and she’ll never do that. It was absolutely genius.”

Daldry remains guarded about his meetings with the Queen – “I’ve had lunch and things with her” – but clear about the impact of this brilliant joke. “I think it was particularly useful that she could show that sense of humour after the Diamond Jubilee which was – correctly – so stately and respectful. She made last year incredibly powerful, because she broadened and reinvented her appeal.”

Morgan thinks the process of reinvention goes further back than that. “There came a point where she became the nation’s grandmother – and at that moment a sort of truce was declared. At a certain level, antimonarchist or republican thought was just suspended out of a sense of decency. And that has spilt over into something rather deeper.”

Mirren points out that The Queen also had something to do with that redefinition of feeling. “It was all there to be seen but people were refusing to see it.” From her point of view, returning to the character after a six-year gap, she notices how very much more material about the Queen there now is to study; the home movies released by Prince Charles, for example, and the many documentaries made to mark the Diamond Jubilee. “Something has really changed,” she says.

On stage, she does not portray the Queen at just one time of her life, but back into her youth. It means she has to age both down and up with the help of wigs, costumes and subtle shifts in diction. “Her voice has changed so hugely that if I can somehow access that young voice, that does so much,” says Mirren. It is, she adds with a wry smile, “easier to get older than younger, unfortunately.”

But although she feels more exposed on stage – “you don’t have the benefit of the editing room” – her starting point for understanding the character of the Queen has remained the same snippet of film she held in her mind when she played her on screen. It shows Elizabeth as a girl of 12 or 13, getting out of a big black car, and greeting, with great seriousness and care, a crowd of men in dark clothes that have come to meet her.

“To me that said everything about her character,” says Mirren. “You have got to do it right. Even from the age of 12. Don’t make a fuss and do it right.”

She is understandably wary of being too closely identified with the Queen. In person, she is absolutely nothing like either the monarch – or her own slightly glacial public persona. The beauty and elegance are there, but she has a sharp wit, direct opinions, and a mischievous humour: she prods Daldry in the ribs when he fails to say how much he was looking forward to working with her; she roars with laughter when Morgan teases her about taking a chauffeur-driven car to work while imagining the Queen separated from the general public.

It is important to remember, she says, that The Audience is a play not a documentary. It will reveal things about the monarchy and about the characters who become prime ministers, but it should be judged on its imaginative power not its accuracy.

“When I was doing the film, it was an incredible liberation for me the day that I thought, ‘this is a portrait’,” says Mirren. “The Queen has been painted many, many times and it’s the artist’s interpretation of her. I thought, that’s it. I am not impersonating her. Of course, I have got to look like her, move like her, sound like her, but it is my interpretation because we are artists and we are allowed to do that.”

In a sense, too, it is the ultimate unknowability of the Queen which makes her such a fruitful subject. Mirren remembers her mother’s reaction when she had her first period: “She said, ‘don’t worry darling, even the Queen has them’. This Queen is everything. She represents something human, but also iconic and representational. It is amazing.”

Daldry agrees: “We project on to her extraordinary qualities, powers and beliefs. Whether she has them or not doesn’t really matter. What is important is that the British people project onto her perhaps the best of themselves.”

That is, Morgan concurs, exactly why he wanted to write about her once more. “I am really not that interested in her as a person. I am interested in what it is to be British and she happens to be a central component of that, of who we are, and how our country is put together.”

The Audience is at the Gielgud Theatre, London W1, (0844 4825130) from Feb 15 until June 15